Biological safety is a paramount concern in modern society. Testing for biological contamination in foods or other materials has become an important and often mandatory requirement for developers and distributors of food products. Biological testing is also used to identify bacteria or other agents in laboratory samples such as blood samples taken from medical patients, laboratory samples developed for experimental purposes, and other types of biological samples. Various techniques and devices can be utilized to improve biological testing and to streamline and standardize the biological testing process.
A wide variety of culture devices (e.g., petri dishes containing agar media) have been developed. As one example, thin film culture devices have been developed by 3M Company (hereafter “3M”) of St. Paul, Minn. Thin film culture devices are sold by 3M under the trade name PETRIFILM plates. Thin film culture devices can be utilized to facilitate the rapid growth and detection of bacteria or other biological agents including, for example, aerobic bacteria, E. coli, coliform, enterobacteriaceae, yeast, mold, Staphylococcus aureus, Listeria, Campylobacter, and the like. The use of PETRIFILM plates, or other growth media, can simplify bacterial testing of food samples.
Culture devices can be used to enumerate or identify the presence of bacteria so that corrective measures can be performed (in the case of food testing) or proper diagnosis can be made (in the case of medical use). In other applications, culture devices may be used to rapidly grow bacteria or other biological agents in laboratory samples, e.g., for experimental purposes.
Automated counting systems are also known. They can be separated into two basic categories. The first category includes those systems employing cameras or video equipment in conjunction with hard-wired circuits or digital computers to count the number of colonies in a petri dish. Examples of such systems are described in EP Publication No. 0 301 600; U.S. Pat. No. 3,811,036 to Perry; and French Publication No. 2 602 074.
Video-based systems suffer from a number of disadvantages. The primary disadvantage is the expensive and sophisticated equipment used in such systems to process the raw pixel image produced by the video cameras. To avoid multiple counting of the same colonies such systems typically include processing-intensive labeling schemes requiring relatively powerful computer systems to accurately count of the number of colonies in an acceptable amount of time.
An additional disadvantage is that many of these video-based systems require that the petri dishes be illuminated through their bottom surface which requires a substrate which is light permeable to ensure accurate counting. In other systems, such as that described in EP 0 301 600, the absorbance and transmission of light is used to detect colonies.
The second category of automated counting systems typically uses an array of photodetectors and hard-wired circuitry to perform the counting process. As with most of the video-based systems, the counting systems using photodetectors are also limited by the requirement that the petri dish be illuminated through its substrate to produce an accurate count. As a result, the substrate on which the colonies are contained must be light permeable, which is a particular problem with disposable culturing devices such as PETRIFILM. Examples of such systems are disclosed in U.S. Pat. No. 3,493,772 to Daughters II et al. as well as U.S. Pat. No. 3,736,432 to Sweet.
U.S. Patent Application Publication No. 2005/0053265 by Graessle et al. and U.S. Pat. No. 5,403,722 by Floeder et al. describe a method and apparatus for counting the number of microorganism colonies present on culture devices, such as PETRIFILM.
A need still exists for flexibility and automation in the use of culture devices for detection of microorganisms.